1. Field of the Invention
The present invention relates to a power supply system, a powered device, and a power reception method for correcting an impedance unbalance in a transmission line used to supply power.
2. Description of the Related Art
The Power over Ethernet (PoE, registered trademark) system specified by the Institute of Electrical and Electronic Engineers (IEEE) 802.3af standard is conventionally known as a system for supplying power via a communication line, such as an Ethernet (registered trademark) cable.
In PoE, power sourcing equipment (PSE) supplies power to a powered device (PD). The IEEE 802.3af standard specifies a sequence for such power supply. In PoE, the PSE supplies power to the PD through three phases, detection, classification, and power supply. The PD incorporates a signature resistance of 25 KΩ (representative value). In the detection phase, the PSE outputs a low voltage to detect the connection of the PD with the line. The low voltage used for the detection ranges from 2.8 to 10 V, and a current of up to 5 mA is supplied. When the voltage output from the PSE is applied to the signature resistance in the PD, a current corresponding to 25 KΩ flows through the line. After detecting the current value corresponding to 25 KΩ, the PSE proceeds to the classification phase.
In the classification phase, the PSE also detects a predetermined current generated due to a resistance value in the PD to thereby classify the power consumption of the PD. The PSE then proceeds to the power supply phase in which the PSE normally delivers a voltage of 48 V, and the PD is allowed to consume power up to 12.95 W. (see, for example, Japanese Patent Application Laid-Open Nos. 2008-154069 and 2000-134228; “IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications Amendment: Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI)” published by IEEE Standards Association Standards Board and approved on 12 Jun. 2003; and 33C.1.10 Test Procedure PSE-10 (turn on, detection and classification time); for the PoE IEEE802.3af standard, see (http://telec.org/feature/feature13.html).)
There is another conventionally known power supply system that performs remote power supply using two pairs of unshielded twisted pair (UTP) cable used for data transmission. The power supply system gives an alarm upon detection of an abnormal condition of the transmission line, such as polarity reversal. (see Japanese Patent Application Laid-Open No. 2000-134228, for example.)
A transmission line connecting a conventional PSE and a conventional PD may comprise first and second constituent lines Each constituent line may, for example, be a twisted-pair cable. In such a case, a first stray capacitance C1 may be considered to exist between the first constituent line and a frame ground (FG) of the PSE, and a second stray capacitance C2 may be considered to exist between the second constituent line and FG. Usually, there is a relationship that the stray capacitance C1≠the stray capacitance C2. When the relationship the stray capacitance C1≠the stray capacitance C2 is satisfied, the transmission line becomes unbalanced. This causes a noise immunity level to decrease, so that 50/60 Hz alternating current (AC) noise (hum noise) is induced from an alternating current (AC) source in the PSE. The hum noise may also be induced from an AC source in an external device (for example, a picture monitor) connected to the PD (for example, a network camera).
In the power supply system in which the PSE detects the PD, if such hum noise is induced into the transmission line, 50/60 Hz AC noise having an amplitude of xxVpp, for example, is superimposed on a low voltage used to detect the PD (hereinafter referred to as a “detection voltage”).
As the amplitude of the superimposed AC noise increases, a value of the current generated as a result of the PD's reception of the detection voltage exceeds a current value range in which a detection device in the PSE can detect the PD, so that detection failure may occur. Accordingly, the PSE repeats the detection process and cannot proceed to the next phase to start power supply, and the PD is left in an inoperative state.
To address this, Japanese Patent Application Laid-Open No. 2000-134228 discusses to provide a detection device that detects an abnormal condition of a transmission line on the PSE side. When the detection device detects an abnormal condition, an impedance of the transmission line may be adjusted to enable the PSE to detect the PD. However, to install an impedance adjustment device on the power supply unit side, the networking equipment, such as the already-existing PSE and the installation condition thereof, needs to be replaced.
On the other hand, installation of the impedance adjustment device for the transmission line on the PD side does not require such large-scale replacement of the networking equipment. However, if the impedance adjustment device for the transmission line is installed on the PD side to correct an impedance unbalance in the transmission line prior to the start of power supply and enable the PSE to detect the PD, the power necessary for operating the impedance adjustment device needs to be provided before the start of power supply.